RuntimeInformation could be better

[NattyNarwhal]

Mono's implementation is ignorant of anything other than WebAssembly/Windows/macOS/Linux, and x86/amd64

Even with this fixed, you'd need to get AIX/OS400 entries added for OSPlatform in CoreFX, wire it up to Mono, and then add Architecture enum entries for PowerPC.

[NattyNarwhal]

For example: https://github.com/dotnet/corefx/issues/23989

[NattyNarwhal]

Now blocking on https://github.com/dotnet/corefx/issues/30706